UHMW will take its shape back, slides over objects better and doesn't rattle. Most skid plates UHMW or aluminum will have limited access while the skid plate is installed, you don't want a bunch of access holes in your skid plate to snag on everything.
The main advantage of UHMW is how easy it slides over rocks or obstacles and how durable it is. Aluminum tends to gouge, scrape and drag when going over rocks where UHMW doesn't. Aluminum also can crack & rattle. There is also a lot of difference in the different UHMW skid manufacturers skid designs so do your homework. Some are just a cut out of your factory skids which only covers the vitals which is fine if you don't really do anything serious but if plan on actually going off road then you want a full body skid. The full body skid basically gives you a full sheet under your car and minimizes anything that you can get hung up on & again allows you to just slide over everything. Most UHMW manufacturers do have drain/vent holes & access holes for cvt & drain plugs but again they are all different so you would need to check. I personally run Factory UTV skids on 4 of my machines now. They brought UHMW to the UTV market & have a proven record of their product podium in all the toughest races there are. Baja, KOH, BITD and so on.